The present invention relates to hydrostatic transmissions for vehicles, and control systems therefor, and more particularly, to such control systems which permit the vehicle operator to drive the vehicle in the manner of an automobile, such control systems being referred to as "automotive drive controls".
Although the present invention may be used with various types of variable displacement pumps, wherein the displacement of the pump is controlled by a fluid pressure actuated servo mechanism, the invention is especially suited for use with axial piston pumps of the swashplate-type, and will be described in connection therewith.
It has been well known in the art for many years to vary the displacement of an axial piston pump of the swashplate-type by porting fluid under pressure ("control pressure") to either of two servos having pistons attached to the swashplate. The control of such control fluid is governed typically by a control valve having a control spool which has a feedback linkage connected to the swashplate, or to the servos. Such control valves are typically referred to as "manual controllers", because the position of the swashplate ultimately corresponds to the position of a manual control handle.
More recently, there has been an increasing desire on the part of the vehicle manufacturers to enable the vehicle operator to drive the vehicle in a manner which is more familiar to the typical vehicle operator, i.e., in a manner similar to driving an automobile. Therefore, those working in the hydrostatic transmission art have developed "automotive drive controls", whereby the vehicle operator controls the displacement of the pump, and therefore, the speed of the vehicle, by means of the accelerator pedal associated with the vehicle engine.
The provision of such automotive drive controls has generally represented an improvement in the vehicles equipped with such controls, primarily because a vehicle equipped with an automotive drive control can be driven by an operator having a lower level of skill and training than what is required to drive a vehicle on which the hydrostatic transmission includes a conventional manual controller. Furthermore, it is generally agreed that the presence of an automotive drive control reduces the fatigue level of the operator, and permits greater overall productivity on the part of the operator.
In general, automotive drive controls make it possible for the vehicle manufacturer to tailor the vehicle drive performance to the torque curve of the engine. For example, an automotive drive control allows the engine to idle, thus saving engine horsepower and fuel, when power is not needed by the vehicle.
Typical examples of vehicles equipped with an automotive drive control would include industrial vehicles, such as fork lift trucks, and construction vehicles, such as front end loaders. It is quite common for such vehicles to operate in hilly terrain (for example, a front end loader operating in a gravel pit), or to operate in a situation where it is desirable to limit the vehicle's speed when the vehicle is more heavily loaded (for example, a fork lift truck traveling with a heavy load on the forks with the forks raised). One of the shortcomings of the prior art automotive drive controls has been an inability to operate in a desirable manner under conditions such as those described above.
In a typical automotive drive control, control pressure to the servos of the pump is proportional to engine speed, with engine speed typically being determined by sensing a pressure differential across a fixed orifice disposed on the outlet side of the source of control pressure (for example, the charge pump). Unfortunately, as the vehicle passes over the top of a hill, and begins to travel downhill, engine speed will tend to increase because of the reduced load on the engine, thus increasing control pressure and pump displacement which, in turn, further increases vehicle speed at a time when increasing the vehicle speed is clearly not desirable.
The "overspeed" problem described above is made worse by the fact that most vehicles equipped with automotive drive controls are also equipped with diesel engines. As is well known to those skilled in the art, the braking torque of a diesel engine is typically only about 30% of the driving torque of the engine. In other words, a typical diesel engine does not provide the type of dynamic braking which would help to alleviate the "overspeed" problem. Furthermore, in the downhill condition described above by way of example only, the momentum of the vehicle will cause the motor of the hydrostatic transmission to be driven and act like a pump, pumping fluid to the pump of the hydrostatic transmission, which will then act as a motor, driving the vehicle engine even faster.